The headword is a masculine noun in the nominative singular; see generally LSJ s.v. and cf. tau 129, tau 133, and tau 134. The entry appears to be generated from specific usages of this noun for surfaces, broad and flat, evocative of a wicker-work pad or matting: see next note.
[1] The headword is identically glossed in the Synagoge (tau37), Photius' Lexicon (tau66 Theodoridis), and Etymologicum Magnum 747.7 (Kallierges), which cites the rhetorical lexica of Aelius Dionysius and Pausanias (the grammarian). The accusative singular of the headword occurs at Homer, Iliad 11.377 (web address 1), again 388, where Paris shoots an arrow which pierces Diomedes [Author, Myth] through the flat of his foot. The Suda might well be drawing from a scholion (= D scholia) to this Homeric passage which glosses tarso/n as to\ platu\ tou= podo/s, the flat of the foot. Cunliffe (p. 373) notes that the upper flat surface of the foot (formed by the metatarsal bones) is meant.
[2] Similarly in Hesychius s.vv. tarro/s and tarsou\s kala/mwn.
[3] Thucydides 2.76.1 (web address 2), describing an episode in the Peloponnesians' siege of Plataiai (Barrington Atlas map 55 grid E4) in 429 BCE; cf. epsilon 1282 and epsiloniota 109. [In her critical apparatus Adler reports that mss AF give the participle as e)nei/lontes; also that, for the finite verb, mss VM have e)se/balon (aorist, inserted) and ms F e)pe/ballon (were throwing onto).]